A standard turbomachine has a rotor shaft mounted in a housing, at least one radial impeller arranged on an end of the rotor shaft in a respective impeller housing and having a seal assembly between a flow region of the impeller housing and a free space surrounding the rotor shaft inside the housing. The seal assembly in turn has a plurality of seals spaced apart from one another in order to separate the flow region from the free space. The radial impeller serves to expand or compress a fluid so that the flow regions inside the impeller housing, in particular a flow region radially adjacent the impeller, are at a pressure different from that of the rotor shaft inside the free space surrounding the housing, typically higher.
In order to avoid unwanted fluid flow through the housing and to avoid pressure losses that reduce the efficiency of the turbomachine, a seal assembly on an axially extending part of the shaft is usually provided between the flow regions of the impeller housing on the one hand and the free space inside the housing on the other. From practical experience it is also known to arrange seals in the form of brush seals, slip ring seals or labyrinth seals between the rotor shaft and housing on an axially extending part of the shaft. To make this possible, the rotor shaft must have a sufficient length inside the housing. Furthermore, bearings and optionally an electrical machine in the form of a motor or a generator must be provided in the housing. Accommodating all this structure—electric machine, bearings, seals—on the shaft requires it to be quite long.
A turbomachine that has a circumferential brush seal in a radial gap between the back of a radial impeller and an associated housing surface for expanding or compressing a fluid in the form of a gas or vapor is disclosed in DE 10 2004 041 439. In an embodiment with the characteristics described in the introduction, according to which the seal assembly has a plurality of seals that are spaced apart from one another, in addition to the circumferential brush seal, a labyrinth seal is provided on an axially extending shaft part so that a flow region laterally adjacent to the radial impeller is separated from the free space surrounding the rotor shaft inside the housing by the brush seal in the radial gap and by the labyrinth seal on the axially extending part of the shaft.
Turbomachines provided on the rear face of the radial impeller with a fluid chamber that can be pressurized with a compressed fluid in order to compensate for axial thrust forces are disclosed in EP 1 281 836 [U.S. Pat. No. 6,616,423] and DE 10 2006 049 516 [U.S. Pat. No. 8,113,798]. The supporting forces from the intermediate space as a buffer can be varied by feeding in and pumping out the compressed fluid. A seal assembly between the rear of the radial impeller and the housing on the one hand and an axially extending part of the rotor shaft on the other is necessary in order to define this intermediate space.
Due to the high rotational speeds, the problem frequently arises when operating turbomachines that the natural frequency of the components, in particular of the rotor assembly, must be taken into account. When the operating frequency in the turbomachine lies in the region of critical natural frequencies, there is a risk of increased vibration that can lead to increased wear or even to damage to the turbomachine. It is therefore known to avoid the regions of specific natural frequencies when operating a turbomachine.